Selective harmonic mitigation‐pulse‐width modulation technique with variable DC‐link voltages in single and three‐phase cascaded H‐bridge inverters

Abstract

Multilevel converters have received increased interest recently as a result of their ability to generate high quality output waveforms with a low switching frequency. This makes them very attractive for high power applications. A Cascaded H-Bridge converter is a multilevel topology which is formed from the series connection of H-Bridge cells. Optimized pulse width modulation techniques such as Selective Harmonic Elimination (SHE-PWM) or Selective Harmonic Mitigation (SHM-PWM) are capable of pre-programming the harmonic profile of the output waveform over a range of modulation indices. Such modulation methods may however not perform optimally if the DC links of the Cascaded H-Bridge Converter are not balanced. This paper presents a new SHM-PWM control strategy which is capable of meeting grid codes even under non-equal DC link voltages. The method is based on the interpolation of different sets of angles obtained for specific situations of imbalance. Both simulation and experimental results are presented to validate the proposed control method. I. INTRODUCTION Multilevel converters have become the focus of research in recent years as a result of their suitability for high power applications [1]. Amongst the available topologies are the Neutral Point Clamped (NPC), Flying Capacitor (FC) and Cascaded H-Bridge converters (CHB) [2]. The latter is constructed from a series cascade of three-level H-bridges. This connection enables the converter to produce high quality, high voltage waveforms whilst utilizing low or medium voltage switching devices. This functionality makes this converter an attractive option for grid connected applications such as Uninterruptible Power Supplies, Static VAR compensators, Series and Shunt Compensators etc. [3]-[10]. The use of power electronic converters at high power levels usually demands a reduction in switching frequency in order to ensure that losses caused by the imperfect nature of practical Manuscript

Non-equal DC link Voltages in a Cascaded H-Bridge with a Selective Harmonic Mitigation-PWM Technique Based on the Fundamental Switching Frequency

“…These techniques employ separate DC link voltages that are supplied by front-end converters. Non-equal DC link voltages can significantly increase the range of solution sets in optimal modulation techniques [30], [31]. More importantly, by using non-equal DC link voltages, solutions can be obtained in a continues way in the whole range of modulation indices.…”

“…More importantly, by using non-equal DC link voltages, solutions can be obtained in a continues way in the whole range of modulation indices. A comparison between employing non-equal DC link voltages in both the SHE-PWM and SHM-PWM methods with the same number of switching transitions was presented in [31].…”

“…Moreover, a maximum number of low order harmonics will be mitigated by the proposed SHM-PWM. To reach these goals, the proposed method employs non-equal DC link voltages as degree of freedom to solve the corresponding non-linear SHM-PWM equations [30], [31].…”

A four-quadrant modulation technique for Cascaded Multilevel Inverters to extend solution range for Selective Harmonic Elimination / Compensation

“…The limitation equation, (11) and (12) can be derived from (2) and (10). The left hand side (LHS) of the equations represent the required phasors need to be compensated, and the right hand side (RHS) of the equations represent compensation capacity.…”

“…Conventional solutions including only adapting the SHE / SHC method in the regions that effective solution exists [1], or sacrificing the number of harmonics to be controlled where effective solution does not exist [5,8]. Some authors proved that non-symmetry modulation [9] and unequal DC-link voltage [10] can extend the solution range, but these method would increase increasing number of equations that need to be solved, and increase the system complexity. Moreover, the extended solution range is still limited.…”